Electrical discharge machining methods for electrically non-conductive materials are known in the prior art. They are used, inter alia, to make bores in structural parts provided with a ceramic coating. Thus in turbine blades, for instance, which have a ceramic heat-insulating layer on a metallic basic element, cooling-air bores are made by spark erosion.
In DE 41 02 250 A1, a method for the electrical discharge machining of electrically non-conductive materials is described in general terms. In this method, the non-conductive material, prior to its machining, is coated with an electrically conductive substance. This layer is used as an auxiliary electrode, which, in the electrical discharge machining, creates an electrical contact with a working electrode. The electrically non-conductive material coated with the auxiliary electrode, and at least that end region of the working electrode which points to the auxiliary electrode and on which, in the machining, the spark discharge is realized, are immersed in a dielectric, which is formed by a fluid such as kerosene or even by a gas.
If a voltage is applied to the arrangement, between the working electrode and the auxiliary electrode a spark discharge is realized and, subsequently, an erosion of the auxiliary electrode as well as of the underlying electrically non-conductive material. At the same time, a part of the dielectric is cracked, whereby carbon or conductive carbides are generated, which are deposited in the form of an electrically conductive layer onto the exposed surface regions of the non-conductive material. The electrically conductive layer which is deposited in this way thus replaces the removed material of the auxiliary electrode and, when the working electrode penetrates into the non-conductive material, creates a conductive connection to the surface of the non-conductive material, so that a continuous machining is possible.
A drawback of this method is that the layer of the electrically conductive substance, forming the auxiliary electrode, has to be laboriously removed after the electrical discharge machining. Additional problems can arise if remaining remnants of the electrically conductive substance interact in such a way with the electrically non-conductive material that they adversely affect the quality and strength thereof. In the worst case, the remnants of the electrically conductive substance react during operation with the electrically non-conductive material and thus cause damage thereto.